| Summary: | Working fluid selection is crucial for organic Rankine cycles (ORC). In this study, the relationship between molecular structure and ORC performance was established based on the quantitative structure–property relationship (QSPR) and working fluid parameterized model (WFPM), from which an ORC working fluid was actively designed. First, the QSPR model with four properties, namely, critical temperature (<i>T</i><sub>c</sub>), boiling point (<i>T</i><sub>b</sub>), critical pressure (<i>p</i><sub>c</sub>), and isobaric heat capacity (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mi>c</mi><mi mathvariant="normal">p</mi><mn>0</mn></msubsup></mrow></semantics></math></inline-formula>), was built. Second, the evaporation enthalpy (<i>h</i><sub>vap</sub>), evaporation entropy (<i>s</i><sub>vap</sub>), and thermal efficiency (<i>η</i>) were estimated by WFPM, and the results were compared with those using REFPROP to verify the calculation accuracy of the “QSPR+WFPM” coupling model. The average absolute relative deviations of evaporation enthalpy and entropy are below 8.44%. The maximum relative error of thermal efficiency is 6%. Then, the thermodynamic performance limit of ORC and corresponding thermophysical properties of the ideal working fluid were calculated at typical geothermal source conditions. Finally, the active design of the working fluid was conducted with the ideal working fluid <i>T</i><sub>c</sub> and <i>p</i><sub>c</sub> as the target. The research shows that C<sub>3</sub>H<sub>4</sub>F<sub>2</sub> and C<sub>4</sub>H<sub>3</sub>F<sub>5</sub> are optimal working fluids at 473.15 and 523.15 K heat sources, respectively.
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